Marcia Aurelia Ceionia Demetrias, better known as Marcia, was the mistress and one of the assassins of 2nd century AD Roman Emperor Commodus from 182–93. Marcia was the daughter of Marcia Aurelius Sabinianus, a freedwoman of the co-emperor Lucius Verus.

Commodus' favorite mistress

Before Marcia was Commodus' mistress, she was the lover and mistress of one of his cousins, Senator Marcus Ummidius Quadratus Annianus, and subsequently a wife of his servant Eclectus. In AD 182, Lucilla,
the sister of Commodus, convinced Marcia to join in a plot with
Quadratus to kill Commodus. The plot was discovered and both Lucilla and
Quadratus were executed. Marcia managed to escape charges, and after
Commodus' wife Bruttia Crispina was exiled and murdered due to adultery, Commodus chose not to marry again and took Marcia as his concubine.
Marcia was most likely Christian and persuaded Commodus to adopt a policy in favor of Christians, and kept close relations with Victor, Bishop of Rome. After Pope Victor I gave her a list she had asked for including all of the Christians sentenced to mine works in Sardinia, she convinced Commodus to allow them to return to Rome.
Despite the fact that Marcia was not Commodus' legal wife, he treated
her like one and was thus greatly influenced by her. The inscription
found in Anagnia
testifies that the local city council decided to build a monument,
commemorating particularly the restoration of baths on her account.

Assassination of Commodus

To
celebrate the Roman New Year in AD 192, Commodus decided he wanted to
make an appearance before the Roman people not from the palace in
traditional purple robes, but from the gladiator's barracks,
escorted by the rest of the gladiators. After telling his plan to
Marcia the night before, she begged him not to behave so carelessly and
bring disgrace to the Roman Empire. Commodus, upset by Marcia's reaction, then told his plan to Aemilius Laetus, the Praetorian Prefect, and Eclectus, his servant. After they, too, tried to dissuade him, he became furious and put their three names on a proscribed list of people to be executed the next morning, including names of prominent senators.
While Commodus was taking a bath, his favorite servant boy
Philcommodus (whose name is a symbol of Commodus' fondness for the boy)
found the tablet on which the list was written and ran into Marcia while
he was holding the tablet.
Marcia took it from him, thinking she was protecting a document from
potentially being ruined, and in horror saw her name at the top of the
list. According to Herodian, she cried out, "Well done, indeed,
Commodus. This is fine return for the kindness and affection I have
lavished on you and for the drunken insults which I have endured from
you all these years. A fuddled drunkard is not going to get the better
of a sober woman".
She then gathered Praetorian prefect
Laetus and Eclectus, Commodus' chamberlain, and the three of them
decided they had to kill Commodus in order to save their own lives.
Usually Marcia would give the emperor his first drink after his bath so
he could have the pleasure of drinking from his lover's hand. It was
easy, then, for her to mix poison into the wine she gave to Commodus
after his bath. After drinking the wine, he became so ill that his
vomiting would not cease. The three conspirators were afraid he would
expel all the poison, so they ordered Narcissus, a young athlete, to strangle Commodus for a large reward.
After Commodus was murdered, Marcia and Eclectus were married, but she was soon after killed by Didius Julianus in AD 193.